Past results strongly indicate that exposure to 12 atmospheres absolute (ATA) helium oxygen gas mixtures (heliox) represents a direct antagonist of ethanol's initial actions which can be used as a tool for investigating the molecular and cellular events leading to acute ethanol intoxication, tolerance, and physical dependence. The present proposal is based on this premise. the goal of Specific Aim 1 is to use low level hyperbaric exposure as a novel means to test the hypothesis that the GABAA- benzodiazepine receptor complex (GBRC) represents a site of ethanol's initial action. This will be accomplished using in vivo and in vitro approaches to test three predictions based on this hypothesis: 1) Low level hyperbaric exposure will antagonize all of ethanol's behavioral effects (in mice) which have been linked to the GBRC by classical pharmacological investigations, 2) Low level hyperbaric exposure will antagonize the effects of ethanol on GBRC in vitro and 3) Differences between genotypes in sensitivity to antagonism of ethanol's effects on GBRC function in vitro will parallel sensitivity to antagonism by pressure of ethanol's behavioral effects. The first prediction will be tested by examining the effects of hyperbaric exposure on the anticonvulsant, anxiolytic and memory impairing effects of ethanol in this strain. These studies will be conducted in C57 mice due to the amount of information available regarding the effects of pressure antagonism on ethanol. Predictions 2 and 3 will be tested by examining the GABA-stimulated uptake of 36Cl into the cerebrocortical synaptoneurosomes from C57, SS and LS mice. These mechanistic studies will focus on the GBRC due to the depth of existing evidence implicating this system in mediating behavioral effects of ethanol. The goal of Specific Aim 2 is to test the hypothesis that recently discovered genotypic differences between long sleep (LS) and short sleep (SS) mice in sensitivity to low level hyperbaric exposure can be exploited to help identify the cellular sites and underlying genetic bases mediating selectively bred differences in ethanol sensitivity. This hypothesis will be tested by answering two key questions regarding the characteristics and genetic relevance of these differences: 1) What is the nature and extent of the differences in sensitivity to pressure antagonism in LS and SS mice? and 2) Does the differential sensitivity to pressure antagonism in LS and SS mice represent a genetically correlated response to the selection pressure or is it an accidentally fixed trait? Question 1 will be answered by determining whether the differences between LS and SS mice in sensitivity to antagonism by pressure of ethanol's behavioral effects are quantitative (threshold) or qualitative (all-or-none) and whether they extend across different acute effects of ethanol (loss of righting reflex, locomotor changes, anxiolytic, anticonvulsant and hypothermia). Question 2 will be examined using selectively bred replicate lines and recombinant inbred strains. Overall, this work will contribute to our long-term goal which is to identify specific targets at which therapeutically relevant pharmacological agents can be directed to reduce the social problems, loss of lives, and tremendous economic costs resulting from the misuse and abuse of alcohol.